Genesis:
Standalone: Genesis Plus GX
Retroarch: Genesis Plus GX (Note that the RA core currently has issues with dropping frames, so I recommend the standalone version)

Snes emulation on the Wii is pretty robust. I think Snes9x Next has the best performance (aside from other cores that sacrifice accuracy like Catsfc), but Snes9x GX has a much nicer interface and I'm not sure how much you'll notice the performance difference outside of certain games. N64 emulation is more hit-or-miss. Some popular games will run perfectly like Mario 64 or Mario Kart, while others will barely run at all, or will have errors.

I just launch all of my stuff straight from the HBC so I think this is as much as I can help with.

Genesis:
Standalone: Genesis Plus GX
Retroarch: Genesis Plus GX (Note that the RA core currently has issues with dropping frames, so I recommend the standalone version)

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mGBA has full support for GB/GBC in the latest official release. It was introduced in the nightlies many months ago, though. It's pretty stable right now, the only thing that it's lacking right now is Super Game Boy 1-2.

mGBA has full support for GB/GBC in the latest official release. It was introduced in the nightlies many months ago, though. It's pretty stable right now, the only thing that it's lacking right now is Super Game Boy 1-2.